Heating, ventilation, air conditioning and refrigeration (“HVAC&R”) systems typically include a compressor delivering a compressed refrigerant from a compressor discharge port to a condenser. The refrigerant is then passed from the condenser to an expansion device, from the expansion device to an evaporator, and finally back to the compressor suction port throughout a closed-loop circuit. The thermal load demand on the HVAC&R system may vary and generally depends on a variety of factors including, but not limited to, indoor and outdoor operational environments, thermal load generation in a conditioned space and fresh air circulation requirements for example. At times, there may be a need for a higher system cooling capacity, and hence higher flow of refrigerant circulating throughout the HVAC&R system is required. At other times, a lower cooling capacity, and consequently a lower refrigerant flow, may be adequate to maintain the conditioned space within the comfort zone. To provide sufficient means of refrigerant flow control, some HVAC&R systems use tandem compressors having the same compressor displacement to provide unloading capability by switching off one of the tandem compressors to match the system capacity to the thermal load in the conditioned space. In such systems, the two or more tandem compressors may simultaneously deliver a compressed refrigerant to a downstream heat exchanger, such as a condenser.
Tandem compressor systems are known, wherein separate condensers are associated with each of the compressors, while the compressors are still connected to the same evaporator. Analogously, tandem compressor systems may be connected to separate evaporators, while still being configured to communicate with the same condenser. The last two configurations are typically utilized when either the condensers or evaporators are associated with separate indoor or outdoor environments that may have different operational characteristics. A control for a typical tandem compressor system will operate one or multiple compressors, depending on the system thermal load. Thus, the compressors can be controlled to provide discrete steps in system capacity to optimize system performance at a full load capacity. Alternatively, the system capacity of a tandem compressor system may be varied by using a compressor unloading function. One commonly employed compressor unloading function may selectively deliver at least a portion of partially or fully compressed refrigerant back to a suction line. In this manner, the amount of compressed refrigerant delivered through the HVAC&R system is reduced when a part-load capacity is demanded. However, a tandem compressor system may not yield optimal performance under partial load conditions, because one of the compressors delivers at least a portion of compressed refrigerant back to a suction line, thus wasting the energy used to compress that portion of refrigerant. There is therefore a need for a refrigeration system that can optimize system performance under full load and partial load conditions.